The present invention relates to material elemental analysis, and is particularly concerned with a non-invasive analyzer apparatus and method for determining quantities of particular elements in large quantities of material.
The importance of elementally enriched matter is inestimable because of the products it makes possible . . . copper for wiring; iron ore, bauxite, platinum and many other ores for cars; phosphate, potash and sulfur for growing crops; gold for electronics and decoration; kaolin and titanium for paints; uranium, coal and oil for fuels; and even ferrous, non-ferrous and heavy metals from recycled waste materials. The Special Nuclear Materials like Thorium, Uranium and Plutonium whether contained in mineral ores or purified and buried within commercial materials are also of critical interest to society. Historically in the winning of useful elements from the earth or recycled material streams, physical sub-samples were collected and analyzed in a laboratory for the quantity and quality of the elements that they contained. The methods were initially “wet chemistry” analyzes but evolved to sophisticated techniques like X-Ray Fluorescence (XRF) or X-Ray Diffraction (XRD) analysis. Sub-sample collection, preparation and analysis typically present analysis results >1 hour later and the full results sometimes days after the elemental material passed the sample point. The use of a sub-sample weighing a few grams to represent 1000's of tons of mineral matter is one of the largest sources of analysis error.
Rapid analytical techniques that do not require sampling but analyze 100% of the material in situ or better yet in motion provide more timely minute by minute analytical information that gives the ability to optimize a mineral processing operation via sorting, batching or the blending of raw materials. The ability to know when sufficient elements of interest are present in the raw materials allows the mine operator or waste recycler to divert less economic matter from the process. This cuts the cost of extracting the useful elements and reduces the often hazardous waste products created during the mineral processing. To effectively detect useful elements requires the ability to see inside rocks, gravel and waste streams that often hide the useful elemental material. Some elements associated with bombs, explosives, drugs and other contraband can be hidden within normal materials and require detection through the concealment. This requires a sensor technology that will penetrate raw materials. The XRF and XRD techniques are typically surface or small sample techniques restricted to very small particle sizes. The on-line techniques of prompt Thermal or Fast neutron analysis with the detection of the signature elemental gamma rays can provide penetrations of order 30 cm to see into rocks and waste streams but unfortunately these techniques are not very discriminating and are typically restricted to detection limits greater than 0.01% for the more sensitive elements. A tabular example of the achievable sensitivities in simple material matrices (less than a dozen components) is given in Table 1. [R. J. Proctor, On-Line Prompt Gamma Neutron Activation Analyzers, Process/industrial Instruments and Controls Handbook, Fifth Edition, Gregory K. McMillan (ed), 10.161]
TABLE 1Typical Belt Analyzer Elemental SensitivitySensitivity in Weight % 1Elements   <0.01%Cl, Sc, Ti, Ni, Cd, Hg, Sm, Gd, Dy, Ho0.01-0.1% S, V, Cr, Mn, Fe, Co, Cu, Rh, Ag, In, Hf,Ir, Au, Nd, Eu, Er, Yb, H0.1-0.3%N, Na, Al, Si, K, Ca, Ga, Se, Y, Cs, La,W, Re, Os, Pt, Pr, Tm0.3-1.0%Li, Be, Mg, P, Zn, As, Mo, Te, I, Ta, Pb,Ce, Tb, Lu, Th, U1.0-3.0%C, Ge, Br, Sr, Zr, Ru, Pd, Sb, Tl  >3.0% Other elements1 Three-sigma detection limit in 10 minutes within an elementally simple rock matrix ≧150 mm thick.
In cases where weak elements are to be detected in a material matrix containing stronger more abundant elements or much greater than the a dozen components of a simple matrix the detection limits can easily be degraded by an order of magnitude worse than given in Table 1. As well as poor discrimination the neutron-gamma techniques are relative analysis methods where the elemental signals of interest must be normalized to correct for neutron flux variations in the samples due to matrix changes in moisture, density and neutron poison levels e.g. ppm levels of B, Li & Cd.
Analogous to the atomic resonant fluorescence process in atoms is the nuclear fluorescence process. The atomic process occurs with optical radiation in the electron-volt (eV) energy range but the nuclear process occurs with highly penetrating gamma ray radiation in the millions of electron-volts (MeV) range but unlike optical radiation most materials are mainly transparent to high energy gamma ray radiation. In the same way that atomic resonance is specific to one energy level of one atom the nuclear resonance is specific to one energy level of one nucleus. The technique of Gamma ray Resonance Absorption or Scattering makes use of this process to provide a very discriminating technique for elemental analysis. The main difficulty has always been to make the resonant X or Gamma Ray radiation.
The problem with making resonant X or Gamma ray radiation is that when a radioactive nucleus emits a gamma ray, the resultant gamma ray is no longer resonant with the original nuclear transition because of the Doppler energy loss caused by the recoil of the emitting nucleus. Also for a gamma ray to be absorbed it must possess energy large enough to excite a nuclear transition and provide energy for the resultant recoil of the scattering or absorbing nucleus. In some cases of nuclei in a crystalline solid the momentum is totally absorbed by the crystal and the gamma ray has its full energy. This is the Mössbauer Effect. Mössbauer isotopes are good sources of resonant gamma-rays but they only exist for specific elemental isotopes and they do not exist for gamma ray energies much above 0.1 MeV. This restricts Mössbauer sources to crystalline samples with special isotopes e.g. 57Fe, 192Ir. For typical unbound nuclei a resonant gamma ray must have the energy of the nuclear transition plus the energy lost to recoil during the emission and the energy required for absorption/scattering recoil. This energy difference δE from the gamma ray transition energy is given byδE=E2/Mc2[in MeV]Where E is the energy of the emitted photon in MeV, Mc2 is the rest mass energy of the nucleus in MeV. This energy can be provided to the resonant gamma ray by emitting the gamma from a moving nucleus. The required Doppler shift velocity is given byV=c.δE/E=c.E/Mc2[in m/sec]Where c is the speed of light in m/sec. The most efficient method of moving the radioactive nuclei is to physically move the radioactive source because then all emitting isotopes see the same corrective motion and the speed can be varied to allow non-resonant scattering background to be removed.
Titanium rotors have achieved tip speeds up to 1300 m/sec [“Resonance Fluorescence in Re187”, H. Langhoff, Phys. Rev. Vol. 135, No. 1B, 1964]. A DOE ORNL group building composite flywheels for energy storage claims >1500 m/sec peripheral velocities with the limitation being manufacturing quality not the material strengths. Commercial flywheels for energy storage are advanced enough to use magnetic bearings and vacuum chambers and can achieve 1000 m/sec with 24 lb composite rotors and claimed lifetimes of greater than 10 years [Optimal Energy Systems, 2560 W. 237th Street, Torrance, Calif. 90505]. A 1000 m/sec velocity will energy correct an 0.6 MeV gamma ray by 2 eV or a 6 MeV gamma ray 20 eV.
An analysis of various methods for generating more generic resonant gamma rays for the borehole analysis of mineral matter is given by B. D. Sowerby, Nucl. Instr. and Meth. 108, 317 (1973). He concludes that an effective method is a gaseous radioactive source that emits a small fraction (1%) of its gamma rays from nuclei moving rapidly towards the scattering nuclei such that it compensates the Doppler shift. The problems are that the source must be strong and be able to be volatilized into a vapor. High temperature long lived radioactive vapors suitable for faster thermal nuclei are potentially environmentally dangerous and the technique has been tried but it has not been well accepted.
Another method of making resonant X or Gamma ray radiation uses a source of photons of distributed energies such that at least some of the photons possess the correct energy to be resonant with nuclei in the material sample to be analyzed. The application of Bremsstrahlung gamma ray radiation for this purpose is described by W. Bertozzi in U.S. Pat. Nos. 5,420,905 and 5,115,459. One problem is that the photon beam entering the sample material must be very intense to guarantee sufficient numbers of photons that actually satisfy resonance. This intensity is expensive to generate, requires exceptional safety precautions and makes a strong background of non resonant photons to the detriment of the analysis signal to noise. An advantage of this method is that many different elements in a sample material can be excited simultaneously but deconvolving the simultaneous scattered resonant photons from many elements in the presence of intense background radiation typically requires expensive detection systems.